1. Field of the Invention
The present invention relates to a surface light emitting semiconductor laser device employed for optical interconnection, optical exchange, or optical information processing, and to a fabricating method thereof.
2. Description of the Related Art
In recent years, research has advanced on optical interconnections, which aim for remarkable improvements in transmission speed, as information transmitting means of logical circuit elements. As a parallel light source thereof, attention has been paid to a surface light emitting laser (vertical cavity surface emitting laser diode, hereinafter, referred to as xe2x80x9cVCSELxe2x80x9d) in which light emitting elements can be arranged in a two-dimensional manner at a high density. Iga, et al. have conducted advanced research on VCSELs, as described in IEEE Journal of Quantum Electronics, Volume 27, page 1332 issued in 1988.
The latest VCSEL has the structure shown in FIGS. 7A and 7B. That is, a resonator 302 is provided in a vertical direction with respect to a horizontal face of a semiconductor substrate 301. The resonator 302 is formed by: an active layer 303 that confines a carrier to generate a light; a lower reflection mirror 304 formed by a semiconductor multi-layered film; an upper reflection mirror 305 formed by a semiconductor multi-layered film; and a spacer layer 306 for aligning a phase of the light emitted in the active layer with ends of both of the upper and lower semiconductor multi-layered reflection mirrors. As a constituent element other than the resonator, there are provided an upper contact layer 307, an upper electrode 308 that also functions as a laser emitting port, an inter-layer insulating film 310, and a lower electrode 309.
In order to oscillate a laser, it is required to confine a carrier and light in the horizontal direction of the carrier 301 (that is, in a direction parallel to a plane including the substrate). As a means for fabricating a substrate 301 and a narrow structure in the horizontal direction, there are provided a method of fabricating a thin columnar (post) structure of several tens of microns by dry etching to form the post section itself as a current path (single post type); a method of fabricating such a post structure, and then making a portion of an AlAs oxide layer insulated by steam oxidization, thereby limiting a current path (AlAs oxidation type); and a method of forming an insulation region by a proton impeller, thereby limiting a current path (proton impeller type).
At present, it is known that a VCSEL having a low threshold current value and excellent current-light characteristics can be fabricated by AlAs oxidization (Journal of Applied Electronics Physical Properties Study Group, Volume 5, Issue 1, 1999, page 11). Reference numeral 312 in FIG. 8 denotes an AlAs layer; 312A denotes an oxide region; and 312B denotes a non-oxide region. An opening 313 is provided at the upper electrode 308 to form a laser emitting port.
There are the following problems with the foregoing conventional VCSEL.
That is, first, the contact resistance of the upper electrode 308 is large. Second, the light output of the device is gradually lowered as power is supplied, and reliability is very low. It was found that these two problems are caused by a GaAs contact layer being thin (several nm to several tens of nm).
In an end face light emitting laser or in a VCSEL having a wavelength of 850 nm or longer, the thickness of the GaAs contact layer, which is the top surface layer of a substrate contacted by the upper electrode, is usually 50 nm to several hundreds nm. In a VCSEL having a short wavelength bandwidth such as a VCSEL with a 780 nm bandwidth, if the GaAs contact layer is thick, the light absorption in that layer increases, and the threshold current value is significantly increased. Therefore, in order to restrict light absorption and obtain practical characteristics, the thickness of the GaAs contact layer is reduced to about several nm or several tens of nm.
On the other hand, during the processes of fabricating a VCSEL, the GaAs contact layer that is the top surface layer is exposed to a variety of process environments, and is easily damaged. For example, in VCSEL fabrication, several steps of film-depositing, such as film-depositing an SiO film by a CVD method onto the GaAs contact layer surface, are carried out. At this time, separation of As from the GaAs top layer is promoted by exposure to the CVD plasma gas or by heating. The GaAs located at a depth of several tens of nm from the top layer is transformed into a layer having poor stoichiometry, or crystalline defects arise. In the subsequent contact hole forming step, this GaAs transformed layer is exposed to a buffered fluoric acid. In general, although GaAs is not easily etched by buffered fluoric acid, this GaAs transformed layer is etched. Here, in the case where the GaAs contact layer is as thick as one hundred nm, several tens of nm of the damaged layer is merely eliminated. In the case where a contact layer in an VCSEL of a 780 nm bandwidth is originally as thin as several tens of nm, all of the GaAs contact layer is etched and disappears, and the lower AlGaAs layer is exposed.
When an upper electrode is formed on the surface of the AlGaAs layer that appears when the GaAs contact layer disappears, the contact resistance increases, and the voltage (drive voltage) required for driving the device increases. In addition, although an opening of the upper electrode serves as a light emitting port, AlGaAs is produced on the surface of the emitting port. Thus, this AlGaAs reacts with the oxygen or moisture in air, and AlGaAs is oxidized and blackened (this is referred to as a xe2x80x9cAlGaAs blackening phenomenonxe2x80x9d). In particular, this blackening phenomenon rapidly progresses by supplying power. As a result, the laser light to be emitted from the emitting port is absorbed by the AlGaAs oxide layer whose top surface has blackened, and making the light output is lowered.
In general, making the GaAs contact layer thicker prevents the GaAs contact layer from disappearing, but on the other hand, light absorption increases. In addition, after the foregoing processes have been completed, the GaAs is eliminated, the GaAs film thickness deviate from the design value therefor, and the reflectance of the reflection mirror is reduced. As a result, the threshold current value of the device is increased.
Along with the above-described two problems, according to the foregoing conventional VCSEL, there is a third problem that the GaAs contact layer is etched by a resist developing liquid employed in the photolithography step when a contact electrode is formed.
That is, as described above, in a VCSEL with a short wavelength bandwidth such as a VCSEL with a 780 nm bandwidth, in the case where the GaAs contact layer is thickened, light absorption in the layer is increased and the threshold current value of the device is significantly increased. Therefore, the GaAs contact layer is reduced to some nm or several tens of nm.
However, in a VCSEL contact layer employing Au for an electrode metal, in general, the layer is often formed by a method in which a negative pattern resist is formed, the electrode metal is film deposited, and then the layer and the negative pattern resist are lifted off. In this case, as shown in FIGS. 18A to 18C, a GaAs contact layer 401 comes into direct contact with the resist developing liquid, and thus, is etched by the developing liquid (reference numeral 404 in the figure). As in the previously described case, in particular, in the case where the GaAs contact layer is several tens of nm, which is very thin, all of the GaAs contact layer 401 may be etched and disappear depending on the developing conditions, such that a lower layer AlGaAs 402 is exposed. Even if the AlGaAs is not fully exposed, the thickness of the GaAs contact layer differs per processed batch, and dispersion in contact resistance value occurs between VCSELs processed in different batches.
In the case where contact electrodes 405 are formed on the AlGaAs layer surface that appears after this GaAs contact layer has disappeared, the contact resistance increases, and the device driving voltage increases. In addition, in the case where GaAs remains in a very thin film, the film thickness of the remaining film differs depending on the sample, and thus, a dispersion in electrode characteristics occurs.
On the other hand, a portion close to an electrode metal at an opening 406 of a contact electrode is a region that cannot be fully covered with a resist. This region is etched in actual contact with the developing liquid. If GaAs of a partial region 407 in the electrode opening 406 is thus etched, the inside of the opening is easily transformed in the subsequent process. Then, the etching of the top layer progresses or the top layer is oxidized. Moreover, after the device has been completed, the device reacts with the oxygen or moisture in air, and the oxidization of AlGaAs progresses, inducing the same AlGaAs blackening phenomenon as described above. If the blackening phenomenon in the emitting port occurs, the laser light to be emitted from the emitting port is absorbed by the blackened emitting port surface, resulting in lowered light output.
As has been described above, even in a VCSEL of a short wavelength bandwidth in which a contact layer is originally as thin as several tens of nm, such as VCSEL with a 780 nm wavelength bandwidth, there has not yet been provided a vertical resonator type surface light emitting semiconductor laser device having high reliability, low resistance and stable characteristics of avoiding an increase in contact resistance, dispersion in electrode characteristics, and a blackening phenomenon due to damage such as erosion or elimination of the contact layer formed of GaAs, and which is capable of producing a high output of laser light which does not decrease over time.
The present invention has been achieved to solve the foregoing conventional problems and to achieve the following objects.
That is, it is an object of the present invention to provide a vertical resonator type surface light emitting semiconductor laser device with having reliability, and capable of achieving high output at a low resistance, and capable of stably producing laser light whose output does not decrease over time.
It is another object of the present invention to provide a surface light emitting semiconductor laser device fabricating method capable of fabricating a vertical resonator type surface light emitting semiconductor device having high reliability and capable of stably maintaining a high laser output.
As a result of earnest studies on techniques for stabilizing laser output of VCSELs of short wavelength bandwidths, the present inventors found that, as described above, a contact layer positioned at an opening (laser emitting port) for emitting a laser is damaged in the process of fabricating the device itself. The damage affects the intensity and stabilization of the laser output, and thus, it is not preferable to stabilize only by avoiding damage incurred during use after the fabrication process has been completed.
Means for solving the foregoing problems is as follows.
According to a first aspect of the present invention, there is provided a vertical resonator type surface light emitting semiconductor laser device comprising a semiconductor substrate having sequentially provided thereon: a first conductivity type semiconductor multi-layered film reflection mirror; an active layer; a second conductivity type semiconductor multi-layered film reflection mirror; and a contact electrode having an opening for emitting laser light, wherein said opening is covered with a protective film for preventing damage due to fabrication after forming said contact electrode.
According to a second aspect of the present invention, there is provided a vertical resonator type surface light emitting semiconductor laser device comprising a semiconductor substrate having thereon: a first conductivity type semiconductor multi-layered film reflection mirror; an active layer; a second conductivity type semiconductor multi-layered film reflection mirror; a protective film; and a contact electrode having an opening for emitting laser light, wherein said contact electrode is provided so as to be superimposed on said protective film, and said opening is formed on the protective layer.
According to a third aspect of the present invention, there is provided a method of fabricating a vertical resonator type surface light emitting semiconductor laser device comprising the step of forming in order on a semiconductor substrate: a first conductivity type semiconductor multi-layered film reflection mirror; a second conductivity type semiconductor multi-layered film reflection mirror; and a contact electrode having an opening for emitting laser light, wherein at least the opening is covered with a protective film immediately after the contact electrode having said opening has been formed.
According to a fourth aspect of the present invention, there is provided a method of fabricating a vertical resonator type surface light emitting semiconductor laser device, said method comprising the step of forming in order on a semiconductor substrate: a first conductivity type semiconductor multi-layered film reflection mirror; a second conductivity type semiconductor multi-layered film reflection mirror; and a contact electrode having an opening for emitting laser light, wherein a sacrifice layer is deposited on a contact layer by a step which is different from a step of forming a wiring electrode bonded with the contact electrode, and a lift-off resist mask is provided on the sacrifice layer, said sacrifice layer is etched with the resist mask disposed thereon, an electrode material is deposited while said resist mask is disposed on the sacrifice layer, and then, said resist mask is lifted off, whereby said contact electrode is formed.
According to a fifth aspect of the present invention, there is provided a method of fabricating a vertical resonator type surface light emitting laser device, said method comprising the step of: sequentially forming on a semiconductor substrate: a first conductivity type semiconductor multi-layered film reflection mirror; an active layer; a second conductivity type semiconductor multi-layered film reflection mirror; and a contact electrode having an opening for emitting laser light, wherein a protective film forming layer for forming a protective film is formed before shaping said second conductivity type semiconductor multi-layered film reflection mirror and forming the contact electrode.
Hereinafter, operation of the above-described vertical resonator type surface light emitting semiconductor laser devices and fabricating methods thereof will be described.
According to the first aspect of the present invention, there is provided a vertical resonator type surface light emitting laser device, wherein, in a step independent of and different from a wiring electrode for power supply, an electrode (contact electrode) bonded to a second conductivity type semiconductor multi-layered film reflection mirror comprises an opening for laser emission, and further, the opening is covered with a protective layer. Thus, the entire surface of the second conductivity type semiconductor multi-layered film semiconductor reflection mirror can be protected from effects from the exterior during use after fabrication as well as during the fabrication process. That is, a wiring electrode and a contact electrode exist independently, and the wiring electrode is connected to and supplies power to the second conductivity type semiconductor multi-layered film reflection mirror via the contact electrode. A patterned protective film is provided, thereby making it possible to form a protective film at an initial stage of the process.
With this structure, there can be avoided an increase in contact resistance due to damage such as erosion or elimination of the surface of the second conductivity type semiconductor multi-layered film reflection mirror made of GaAs, and a blackening phenomenon causing lowered laser output due to blackening. The device can be made to have low-resistance and stable characteristics, and laser light of a high output which is not reduced over time can be stably obtained. Therefore, a device having high reliability and providing a stable supply (of laser light) can be realized.
According to the second aspect of the present invention, there is provided a vertical resonator type light emitting semiconductor laser device, wherein at least a laser emitting region of a second conductivity type semiconductor multi-layered film reflection mirror can be protected from effects from the exterior during use after fabrication as well as during the fabrication process. That is, with this structure, there can be avoided an increase in contact resistance due to damage such as erosion or elimination of the exposed second conductivity type semiconductor multi-layered film reflection mirror and a blackening phenomenon causing lowered laser output due to blackening. The device can be made to have low-resistance and stable characteristics, and laser light of a high output which is not reduced over time can be stably obtained. Therefore, a device having high reliability and providing a stable supply of laser light can be realized.
According to the third aspect of the present invention, there is provided a method of a fabricating a vertical resonator type surface light emitting semiconductor laser device, wherein a protective film is formed immediately after a contact electrode has been formed on a second conductivity type semiconductor multi-layered film reflection mirror or a contact layer (that is, after at least a first semiconductor multi-layered film reflection mirror, an active layer, a second semiconductor multi-layered film reflection mirror, and the contact electrode have been laminated and before there is formed any structure, other than a sacrifice layer described later, of a plurality of structures finally formed on the second conductivity type semiconductor multi-layered film reflection mirror). Moreover, the subsequent steps are completed without causing the protective film to be fully removed. Thus, damage such as erosion or elimination of a contact layer, which damage may be caused during the fabrication process, and in particular, during the step of forming an insulating film or the step of forming the contact hole, can be prevented in particular.
As a result, there can be constantly produced a vertical resonator type surface light emitting semiconductor laser device (hereinafter, occasionally referred to as a xe2x80x9cdevicexe2x80x9d) that can avoid an increase in contact resistance or a reduction in laser output due to the blackening (blackening phenomenon) of an emitting port, and that provides low-resistance and stable characteristics, and that can stably produce laser light with high output that does not decrease over time.
The reliability of a VCSEL with a short wavelength bandwidth and comprising a thin film contact layer, such as VCSEL of a 780 nm wavelength bandwidth, can be improved.
According to the fourth aspect of the present invention, there is provided a method of fabricating a vertical resonator type surface light emitting semiconductor laser device enabling more stable fabrication of a device that avoids a blackening phenomenon that causes an increase in contact resistance, dispersion in electrode characteristics, and lowered laser output due to blackening accompanying damage such as erosion or elimination of the second conductivity type semiconductor multi-layered film reflection mirror due to contact with a developing liquid employed during the fabrication process (and in particular, during resist developing), which device has low-resistance and stable characteristics, and can stably produce laser light at a high output that does not decrease over time.
As in the third embodiment, the reliability of a VCSEL with a short wavelength bandwidth and comprising a thin film contact layer, such as VCSEL of a 780 nm wavelength bandwidth, can be improved.
According to the fifth aspect of the present invention, there is provided a method of fabricating a vertical resonator type surface light emitting semiconductor laser device, wherein a protective film is primarily provided in a laser emitting region. Thus, as in the third and fourth aspects, there can be more stably produced a device that avoids a blackening phenomenon that causes an increase in contact resistance, dispersion in electrode characteristics, and lowered laser output due to blackening caused by damage such as erosion or elimination of the second conductivity type semiconductor multi-layered film reflection mirror during contact with a developing liquid employed during the fabrication process (and in particular, during resist developing), which device has low-resistance and stable characteristics, and can stably produce laser light at a high output that does not decrease over time.
As has been described above, according to the present invention, there can be provided a vertical resonator type surface light emitting semiconductor laser device having high reliability and capable of producing laser light with a high output due to application of a low voltage under a low resistance, and moreover, capable of producing laser light with a stable output that does not decrease over time.
In addition, according to the present invention, there can be provided a method of fabricating a surface light emitting semiconductor laser device which enables fabrication of a vertical resonator type surface light emitting semiconductor laser device having high reliability and able to maintain a high and stable laser output.